1. Field of the Invention
The present invention relates to an acrylic copolymer composition (hereinafter, designated as an acrylic rubber) which is capable of being vulcanized with sulfur and has excellent processing properties, good water resistance, good heat resistance, and good oil resistance. In greater detail, it relates to acrylic rubbers capable of being vulcanized with sulfur which are produced by radical polymerization of (1) a monomer mixture comprising an acrylic acid ester derivative or a mixture thereof and ethylidene norbornene, (2) a monomer mixture comprising the above described mixture (1) and acrylonitrile, (3) a monomer mixture comprising an acrylic acid ester and vinyl norbornene, or (4) a monomer mixture comprising the above described mixture (3) and at least one selected from monovinyl unsaturated compounds and monovinylene unsaturated compounds.
2. Description of the Prior Art
In general, acrylic rubbers have excellent properties such as heat resistance, oil resistance, weather-proofing properties, ozone resistance and gas-impermeability. Particularly, where heat resistance and oil resistance are required, acrylic rubbers are utilized in place of nitrile rubber from which sufficient results could not be obtained. At present, as acrylic rubbers which have been practically used, those composed of an acrylic acid ester derivative such as ethyl acrylate or butyl acrylate or a mixture thereof as a main constituent and produced if desired, by copolymerizing with acrylonitrile, styrene or methyl methacrylate have been widely used. Furthermore, to facilitate vulcanizing, chlorine type rubbers produced by copolymerizing the above described monomers with 2-chloroethyl vinyl ether or vinyl chloroacetate and epoxy type rubbers produced by copolymerizing the above described monomers with allyl glycidyl ether or glycidyl methacrylate, etc. have been used. However, chlorine type rubbers and epoxy type rubbers have the fault that vulcanizates with good properties can not be obtained if they are not subjected to post-vulcanization for 5 to 24 hours at 150.degree. to 200.degree. C. after vulcanization in a mold.
Further, since acrylic rubbers produced by copolymerizing these chlorine type and epoxy type materials can not be vulcanized with sulfur and/or a sulfur donor accelerator alone, a combination of triethylenetetramine with dibenzothiazyl disulfide and a combination of sulfur and a trimene base (a reaction product of ethyl chloride, formaldehyde and ammonia) are usually used. In some other cases ammonium salts of organic carboxylic acids, dithiocarbamates or thiourea derivatives, etc. are used as vulcanizing agents. However, when amines are used as the vulcanizing agent, the level of sanitary conditions of the work room decreases and the resulting acrylic rubber goods can not be used in the medical and food industries. In other words, amine vapor generated gives rise to bad influence on workers' bodies, and also when the resulting acrylic rubber containing unreacted amine is used as a conveyor belt for transportation of foodstuffs, the amine remaining unreacted generates amine vapor resulting in the contamination of foodstuffs. In addition, there is the disadvantage that satisfactory properties often can not be obtained if the rubber is not subjected to post-vulcanization for 5 to 24 hours at about 170.degree. C. after vulcanization at the above described temperature for 5 to 40 minutes. Furthermore, amines cause corrosion of an iron mold or of cords of steel wires buried as a tension member in a rubber-timing belt to shorten the life thereof. In vulcanization of acrylic rubbers using a known soap (sodium stearate or potassium stearate)-sulfur although the above described disadvantages caused by vulcanizing using amines can be avoided, vulcanizates having satisfactory properties can not be obtained if the vulcanization thereof is not carried out at a temperature as high as 170.degree. to 190.degree. C. which is higher than the vulcanization temperature range of 130.degree. to 160.degree. C. of diene rubbers such as natural rubber or SBR (styrene-butadiene rubber), etc. using sulfur. Such a high temperature is difficult to obtain using the conventional press-vulcanization with steam, and consequently, they have the same disadvantage in that post-vulcanization is necessary in the above described chlorine type and epoxy type acrylic rubbers.
As the result of research on improving these disadvantages it has been found that the vulcanizing using sulfur can be carried out by bonding the main chains of acrylic rubbers with saturated hydrocarbons and introducing double bonds into the side chains of acrylic rubbers. Thus, the synthesis of acrylic rubbers having the many characteristics of vulcanizates with sulfur without loss of heat resistance and oil resistance of the acrylic rubbers has been accomplished. Namely, in the rubber rheometer cure curve, there is a range in which the curve rises rapidly with a sharp gradient after a suitable induction time and then a flat plateau range (plateau effect). Thus the processing ability and quality uniformity can be improved and the post-vulcanization required in the prior chlorine type or epoxy type vulcanization system need not be carried out. Further, the heat resistance and the oil resistance of such sulfur-vulcanized rubbers can be easily improved by blending with such rubber. Moreover, in producing a co-vulcanized product by combining different rubbers and vulcanizing by heating under pressure, which is often used industrially, it is necessary to make the vulcanization rates of both rubbers equal. Accordingly, if both rubbers belong to the same or an analogous vulcanization system, the vulcanization rates can be easily adjusted so as to be equal to each other, and, of course, additional high adhesive strength can be easily obtained. Accordingly, development of acrylic rubbers which can be vulcanized using sulfur is industrially important.
As acrylic rubbers already developed for improving the above described disadvantages, those acrylic rubbers produced by copolymerizing an acrylic acid ester with tetrahydrobenzyl acrylate (U.S. Pat. No. 3,497,571), an acrylate of hydroxydicyclopentadiene (Japanese Pat. Publication No. 7893/1972), dicyclopentadiene (British Pat. No. 1,062,969 and U.S. Pat. No. 3,402,158) or methyl cyclopentadiene (U.S. Pat. No. 3,487,054), etc. as a third component are known. However, in these cases, since the ester residues which make sulfur vulcanizing possible are combined with a polymer main chain of the polymer by an ester group, these rubbers have the disadvantages that they are hydrolyzed, other faults are that they are vulcanized at a very slow rate or that they adhere to rolls during rolling operations. In order to eliminate such faults, copolymerization with a small amount of a difunctional diene compound is necessary, and consequently, satisfactory products have not been obtained until now.